1. Field of the Invention
The present invention relates to thin-film resistors and, more particularly, to a method for forming a thin-film resistor.
2. Description of the Related Art
A resistor is a common circuit element that provides a specified electrical resistance under specified conditions. Electrical resistance, in turn, is defined as the ratio of the potential difference between the ends of a conductor and the current flowing through the conductor.
A thin-film resistor is a type of resistor that is used with integrated circuits and, as the name suggests, is formed from a thin layer of resistive material. Numerous resistive materials, including lightly-to-heavily doped polysilicon, silicon chrome (SiCr), nichrome (NiCr), tantalum, and cermet (Cr--SiO), have been used to form thin-film resistors.
The performance of thin-film resistors is defined by a number of parameters which include the resistor value (the resistance that is supposed to be provided by the resistor), the resistor tolerance (the extent to which the resistance may deviate from the resistor value), and the temperature coefficient of resistance (TCR) (the amount the resistance changes with changes in temperature).
It is also important that similarly formed resistors have similar resistances (known as value matching), and similar variations with changes in temperature (known as tolerance tracking). Another parameter, known as an end effect, is a measure of a change in the length of the thin-film resistor that results from metalization spiking into the thin-film resistor.
FIGS. 1A-1H show cross-sectional views that illustrate a process for forming a conventional thin-film resistor. As shown in FIG. 1A, the method begins with a conventionally formed wafer 100 that includes a semiconductor material 110, such as an epitaxial layer or a substrate, and a layer of oxide 112 approximately 5,500 .ANG. thick which is formed on the surface of material 110. In addition, wafer 100 also includes a surface contact region 114.
From this point, as shown in FIG. 1B, a layer of aluminum 116 is cold deposited over oxide layer 112 and material 110 in contact region 114. Following this, a first mask 120 is formed and patterned on the surface of aluminum layer 116 to define a resistor region 122 on the surface of oxide layer 112.
Once mask 120 has been patterned, as shown in FIG. 1C, the unmasked regions of aluminum layer 116 are etched until aluminum layer 116 has been removed from resistor region 122 on the surface of oxide layer 112. After this, mask 120 is removed.
Next, as shown in FIG. 1D, a thin-film layer of silicon chromium 124, is deposited over aluminum layer 116 and resister region 122 on the surface of oxide layer 112. The film composition of the silicon chromium is approximately 72% silicon and 28% chromium.
Following this, a second mask 126 is formed and patterned over thin-film resistive layer 124 to define a plurality of resistors 130. Once mask 126 has been patterned, as shown in FIG. 1E, the unmasked regions of thin-film resistive layer 124 are etched until the unmasked regions of thin-film resistive layer 124 have been removed.
After this, as shown in FIG. 1F, aluminum layer 116 and mask 126 are removed. Next, as shown in FIG. 1G, a second aluminum layer 134 is cold deposited over oxide layer 112, resistors 130, and material 110 to form an interconnect. Next, a third mask 136 is formed and patterned on interconnect layer 134 to define metal interconnect tracks.
Once mask 136 has been patterned, as shown in FIG. 1H, the unmasked regions of interconnect layer 134 are etched until the unmasked regions of interconnect layer 134 have been removed. After this, mask 136 is removed.
Although the above-described process produces thin-film resistors which are adequate for the needs of current generation products, future products are expected to require thin-film resistors which have a greater precision than those currently being produced. Thus, there is a need for a thin-film resistor which has greater precision than current generation thin-film resistors.